Lanthanide complexes have an established place as catalysts and reagents in organic synthesis. An attractive feature of lanthanides in catalysis is the high degree of lability of Ln-to-ligand bonds which allows rapid turnover rates in catalytic reactions. Many of the reactions described in this chapter can be catalyzed by non-lanthanide complexes, but usually turnover rates are much lower and sometimes stoichiometric quantities of catalyst may be required. The use of lanthanide catalysts in polymerization reactions has been reviewed by H. Yasuda and E. Ihara. Lanthanide bis (pentamethylcyclopentadienyl) hydrides, usually generated by hydrogenolysis of the corresponding alkyl complex, show remarkable activity in the hydrogenation of terminal and internal alkenes. A range of lanthanide complexes have been investigated as catalysts for the hydrolytic cleavage of RNA as reviewed by J. R. Morrow et al. Lanthanide alkoxides are soluble and labile Bronsted bases, and are effective catalysts for living polymerization of lactones.